1. Field
The present embodiments relate to a polysilicon layer, a method of crystallizing a silicon layer, a thin film transistor (TFT) using the polysilicon layer, and an organic light emitting display device including the polysilicon layer.
2. Description of the Related Technology
A thin film transistor (TFT) is a type of an electric field effect transistor (FET) prepared by using a semiconductor thin film on an insulting support substrate. Like an electric FET, a TFT can include three devices, e.g., a gate, a drain, and a source, and performs a switching operation. The TFT performs the switching operation by adjusting a voltage supplied to the gate so as to turn a current flowing between the source and the drain on or off. TFTs are used in a sensor, a memory device, an optical device, or the like, and can be used as a pixel switching device or a driving device of a flat panel display.
Currently, commercialized products, such as lap tops, personal computers (PCs), monitors, televisions (TVs), and mobile devices, mostly use an amorphous silicon TFT (a-Si TFT). Amorphous silicon does not have a regular atom arrangement like a crystal, and has a short distance order but not a long distance order. The amorphous silicon is mostly used in TFTs since it is easily deposited in a large area and is easily prepared on a glass substrate at a low temperature. However, as a size and quality of a display increase, a device needs to operate at a high performance. Accordingly, a high performance TFT having higher electron mobility than the amorphous silicon TFT having electron mobility of about 0.5 to 1 cm2/Vs, and a method of preparing the high performance TFT are required.
The polysilicon TFT (poly-Si TFT) has superior performance to a conventional a-Si TFT. Since the poly-Si TFT has electron mobility of tens to hundreds of cm2/Vs, the poly-Si TFT may be built in a data driving circuit or peripheral circuits that require high mobility. Also, since a size of a channel of a transistor may be decreased, an aperture ratio of a screen may be increased. Also, since a wiring pitch, which is used to connect the data driving circuits according to an increase in pixel numbers, is not limited due to the built-in data driving circuit, high resolution is possible, a driving voltage and power consumption may be reduced, and a device's characteristic deterioration may be very low.
A method of preparing polysilicon may be classified into a low temperature process and a high temperature process, according to a process temperature. Here, the high temperature process is performed at a particular temperature or above when an insulating substrate deforms, and thus an expensive quartz substrate having high thermal resistance is used instead of a glass substrate. Also, a polysilicon thin layer formed according to the high temperature process has low quality crystallization, which results in high surface roughness or minute grains.
The low temperature process is a technology of crystallizing amorphous silicon into polysilicon, and excimer laser crystallization (ELC), crystallization using a metal as a catalyst, etc. are being studied. Here, in an ELC process, a pulse shape laser beam is irradiated onto a substrate so as to melt and coagulate amorphous silicon in units of nano seconds. However, such an ELC process is expensive, takes a long time, and is inefficient.
FIGS. 1A and 1B are conceptual diagrams illustrating crystal growth characteristics of silicon in methods of crystallizing silicon by using a metal as a catalyst. FIG. 1A shows the crystal growth characteristics of silicon according to metal induced crystallization (MIC), and FIG. 1B shows the crystal growth characteristics of silicon according to metal induced lateral crystallization (MILC). In the MIC, a relatively large amount of catalyst metal is deposited on amorphous silicon and is crystallized at a high temperature, and as shown in FIG. 1A, small linear polysilicon is grown randomly. In the MILC, a metal used as a catalyst is arranged in a line on amorphous silicon by using a mask and then deposited, thereby growing polysilicon in one direction as shown in FIG. 1B.